1. Field of the Invention
The invention relates to methods and associated instrumentation for stereotactically locating the site of a prosthetic implant in a body, and in particular to methods and associated instrumentation for stereotactically locating and preparing the site for an intervertebral endoprosthesis, and to precisely implanting the endoprosthesis.
2. Description of Related Art
The proper location of any endoprosthetic implant is a key element of the success of the implantation procedure in improving patient quality of life. In spinal implants, for example, properly locating the endoprosthesis ensures among other benefits that the patient will enjoy the full range of motion offered by the implant.
The use of intervertebral implants (arthroplasty) has, in recent years, attained increasing acceptance as a preferable alternative to spinal fusion (arthrodesis) as a method for treating patients where discectomy is indicated. This is in part due to recent advances in implant technology, and in part due to the increasing appreciation of the advantages provided by implantation, including increased range of motion, decreased post-operative damage to adjacent intervertebral discs (which can result from the decreased range of motion at the level of the fusion), decreased risk of harvest site morbidity, etc. This increasing acceptance seems likely to continue for the foreseeable future, and more and more implantation procedures will likely be performed.
Surgical implantation of prostheses into the intervertebral space may be done using a posterior approach or an anterior approach. The posterior approach has the disadvantage of requiring dissection of muscle tissue of the back, which causes patient discomfort and increased healing time. An anterior surgical approach is often preferable for this and other reasons, and is quite practical for implantation between cervical vertebrae, in particular. Irrespective of the surgical approach, precisely locating the desired implant position, and precise positioning of cavity preparation tools and of the implant at that location are essential. Errors in positioning of the implant or in positioning of the devices used to prepare the intervertebral space to receive the implant can be catastrophic to the patient, given the proximity of the surgical site to the spinal cord, vertebral arteries, etc.
Similar considerations apply in other areas of surgery, in particular in neurosurgery. For example, during surgery on the brain, the surgeon often anchors a frame to the sides of the patient's head, which provides constant reference points during surgery, irrespective of how the patient's head or neck is positioned or moved during the procedure. This level of stereotactic precision in location and placement is also desirable for other medical and surgical procedures, such as in spinal surgery, where the margin for error is very small due to the proximity to the spinal cord and other neuro and vascular structures. However, in most procedures used for spinal surgery, as well as surgery to other body parts, it is impossible (or at least impracticable or undesirable) to secure an external frame to the patient's skeleton or soft tissues to provide constant reference points.
Electronic systems exist for stereotactic positioning of medical instruments during surgical procedures. However, these systems are extremely expensive, requiring significant computing power, are highly complex, and require specialized software, and are not always available, particularly in smaller health care systems or in less developed countries.
Accordingly, there remains a need in the art for a method and apparatus for stereotactically locating targeted implantation positions, for precisely positioning tools for preparing the implantation site, and for precisely inserting the implant in the desired position that is simple, safe, that does not require expensive electronic or computerized tracking of medical instrumentation, and that can be used with conventionally available imaging technologies.
Furthermore, there is a need in the art to provide a method and instrumentation that will allow a surgeon to revise two fused vertebra and interpose an articulating implant therebetween.
Examples of procedures for introducing intervertebral implants are described in U.S. Pat. Nos. 5,674,296 and 5,865,846, the entire contents of each of which are hereby incorporated by reference. These patents generally disclose the steps of determining which size and shape prosthetic vertebral body disc unit a patient needs and implanting that prosthetic into the patient's spine. More specifically, the surgeon or medical technician determines the size, shape, and nature of a patient's damaged vertebral body by viewing images of the patient's spine, such as radiographs, CT and/or other MRI scans. Based upon that information, an appropriate size prosthetic disc unit is constructed by a specialized laboratory in conformity with the information provided by the surgeon or medical technician. The prosthetic units are described as having concaval-convex elements.
When the units are received, the patient is prepared and the damaged natural spinal disc material is removed. The surgeon forms holes in the bone structure using a measuring instrument centered in the excavated natural intravertebral disc space. These holes are tapped to form female threads and anchors are implanted therein to form an imaginary platform of reference points with respect to the patient's spine. A bone surface machining jig is then affixed to the anchors and used to form the desired concave surfaces on the inferior and superior surfaces of opposing vertebral bodies, using a milling head or bit of a predetermined size.
After the bone surface machining jig is removed, a prosthetic disc unit, having corresponding concaval surfaces, is inserted between the distracted and milled vertebral bodies. The concaval-convex elements are then attached by the same anchors to the bone, insuring a mate between the bone surface and the concaval-convex elements.
While these methods are certainly sufficient to achieve a successful intervertebral implantation, there remains a need for improved techniques and instruments that provide even more precise localization, such as improved stereotactic location of the desired site of the implant, the precision positioning of milling, burring, and other tools and instruments for conducting the procedure, and the implantation of the endoprosthesis into the prepared site.